Conventional methods of estimating road surface friction may use wheel slip computed from non-electric power steering (EPS) sensor signals. Different approaches for estimating a change in the road surface friction include use of differences in the wheel velocities and the wheel slip, use of vehicle yaw and lateral acceleration sensors, use of optical sensors at the front of a vehicle which use reflection from the road surface to estimate the road friction, use of acoustic sensors to detect tire noise which gives information about the surface, and use of sensors at the tire threads to measure stress and strain which may be referred back to a surface friction.
EPS sensor signals may also be used but detections are made after the steering wheel is in a steady state condition. As such, some subsets of driving maneuvers are excluded because of the steady state requirement. The detection algorithms applied in steady state conditions are binary, tri-state, etc., and limit information available to other vehicle subsystems.
In the case of friction determined based on wheel slip, determinations are costly and dependent on tire signals, or provide a late detection. It is important that the driver gets friction feedback, but friction information is also needed for other functions like EPS feel/torque synthesis, autonomous intelligent cruise control, and collision avoidance systems.